The present invention generally relates to the field of automatic power control (APC) circuitries used e.g. in the analog front end of a mobile transmitter. It particularly refers to different embodiments of a closed-loop power control circuitry integrated into the analog front end of a mobile transmitter and a corresponding method for controlling the radiated power level of an RF signal to be transmitted at the output port of a variable-gain power amplifier integrated into said mobile RF transmitter by performing an additional regulation of the APC loop's reference signal.
In the last few years the demand for high-efficient power control circuitries applied to wireless communication devices has ever increased. One key task in closed-loop power control is the design of analog circuitries to be integrated in the analog front end of a wireless RF transmitter which are used for controlling the output power Pout of an RF signal x(t) to be transmitted over the time t. Ramping too fast results in an unwanted spread of the RF spectrum, and a too slow ramping violates prescribed time constraints. The output power Pout, which is usually supplied by a power amplifier (PA) at the output port of the wireless RF transmitter, is thereby set by an external control voltage Vctrl. The relation between Vctrl and Pout is often nonlinear and influenced by temperature, tolerances, supply voltage, frequency and PA input power. To accomplish a sufficient stabilization of Pout, a power control loop is needed, although some designers still use non-feedback concepts, e.g. by controlling the PA supply voltage. Such a control loop typically comprises an RF detector and a loop amplifier which is supplied by an input signal from a baseband controller. Conventional power control loop designs mainly differ in the respectively applied RF detector, but the loop amplifier also involves interesting design aspects.
One important issue in power control loop design is the dynamic range. For a GSM-based mobile phone the maximum antenna power is 33 dBm, and the minimum power level is 5 dBm. The detector dynamic must be significantly higher, e.g. greater than 34 dB, which is relatively close to what a good diode detector is capable of. Another reason for the need of a large dynamic range is that e.g. in a conventional TDMA-based communication system the power amplifier starts from “power-down” mode in which the RF level is determined by noise and cross talk. This level should be lower than about −48 dBm for the GSM system, which would result in a dynamic range of more than 70 dB. If a control Vctrl voltage is applied to the control input of the power amplifier, the output power Pout increases. But due to the finite detector dynamic, the loop is not locked and at the point the detector responds a large overshoot may occur.
Two issues make power control loop design a difficult task. One is that some power amplifiers are not very fast, which means that there might be a significant delay between a step ΔVctrl at the control input and the corresponding change ΔPout in output power. This limits the speed of the power control loop and can cause instabilities. The second problem is that power amplifiers and many detectors are nonlinear circuit elements. When a power control loop is built with an ideal linear detector and a linear loop amplifier, an ideal power amplifier would have a constant slope dPout/dVctrl but in reality dPout/dVctrl is a function of the control voltage Vctrl which results in a bias-dependent overall loop gain and makes frequency compensation of the feedback system rather difficult. If the loop is stable, however, the circuit might be too slow for some power levels.
The power amplifier is the component of a mobile transmitter that amplifies the RF signal x(t) to be transmitted to the necessary power level Pout needed to drive the transmitting antenna. In most wireless communications systems, the power amplifier is the largest power consumer, usually because the amount of power that needs to be sent to the antenna (the power output) is itself very large. This does not include the total power that is consumed within the power amplifier, just the amount of power which is required to drive the antenna. The total power consumed by the power amplifier is necessarily greater than the power output, as there will always be some power consumed in the active devices and the peripheral circuitry. Since the power output specification itself is often larger than the power consumption of the rest of the blocks in the RF system and the power consumption of such a power amplifier will be higher than the specified power output, the power amplifier is decidedly the major power consumer of the system.
Because the levels of power required to reliably transmit the modulated RF signal x(t) are often relatively high, there is a lot of power consumed within the power amplifier. In many wireless applications, the amount of power consumed by this amplifier is not critical; as long as the signal being transmitted is of adequate power, that is good enough. However, in a situation where there is only a limited amount of energy available, which is not sufficient for the transmission procedure, the power consumed by all devices must be minimized, so as to maximize the length of time for which that energy is available.
The number of different classes of power amplifiers which are used today is too numerous to be counted, and they range from entirely linear to entirely non-linear, as well as from quite simple to inordinately complex. In PA terminology, a “linear” power amplifier is one which has a linear relationship between its input and output. Although a power amplifier may comprise transistors operating in a nonlinear fashion (e.g. in case a FET switches between cut-off and saturation), it can still be considered linear. While nonlinear power amplifiers feature a comparatively high efficiency, their nonlinearity causes the output signal to spread (due to intermodulation products, especially if there is a lot of phase noise in the local oscillator which will cause spreading of the input to the power amplifier).
A typical power amplifier consists of several serial stages. Each stage is usually larger and more powerful than the previous one. As most of the quiescent current is drawn by the high power stages, which are not required for the low output power levels needed for wireless communication, means for bypassing high power stages when they are not required lead to a significant reduction of energy consumption.
Since wireless telephones operate on battery power, it is also desirable that their transmitters operate as efficiently as possible to conserve power and extend battery life. Ideally for W-CDMA systems, such as those governed by the UMTS standard, power amplifier stages should be capable of efficient, linear operation in their required dynamic range. However, the prior art has not yet come close to the ideal, and many wireless telephones are having poor power management now. During low power transmissions, power is wasted by cascaded amplifier stages that are not needed. Consequently, attempts have been made to bypass unused stages.
Under normal operating conditions, conventional wireless RF transceivers devices use an automatic power control (APC) circuit to control the output power of their amplification stages. The APC circuit found in most RF transceivers has an external connection that is intended to be connected to a linear power amplifier. After having detected the power of the modulated RF signal at the output port of the final power amplifier, said signal is converted to a DC voltage and fed back to a variable-gain intermediate frequency (IF) stage in order to keep the final output power constant over a long period of time. As the APC voltage generation is done very early, the gain drift, which is caused by thermal drift, operating voltage deviation, etc., is not compensated by the circuit. Another option is to derive the APC voltage from the drive power of the final amplifier and feed it to the external APC input of the RF transceiver. The theory is that when the power amplifier becomes overdriven, it will produce a negative voltage that is fed back into the transceiver's APC circuitry. This voltage acts as a gain control in the transmit stages of the transceiver which, in turn, automatically lowers the drive power (the transceiver's output power) and limits distortion from the overdriven amplifier.
FIG. 1a shows a schematic block diagram of a conventional APC loop 100a according to the state of the art, which is used for stabilizing the power at the RF output port 114b′ of an analog circuitry realizing an RF signal generator. This circuit can also be used for executing an amplitude modulation (AM). It comprises a frequency synthesizing unit 102′ (FSU), a power divider 106″ (e.g. a directional coupler), which feeds the reflected wave of the modulated RF output signal to a wideband detector diode 108′, and an amplification stage 112′ whose output signal is fed to an electronically controlled attenuator 103′, e.g. an amplitude modulator stage which comprises current-controlled PIN diodes realizing a tunable resistor with hybrid microwave integrated circuit (MIC) technology. In case said RF signal generator is used for sweep-frequency applications, an external detector (not shown) is usually applied in order to keep the power level at the input port of a tested RF unit constant.
FIG. 1b shows a schematic block diagram of a QAM transmitter 100b for a wireless communication device in an EDGE-based communication environment comprising an APC loop 101 according to the state of the art, which is used for stabilizing the power level Pout of the RF output signal at the transmit antenna 110 of the QAM transmitter 100b. Thereby, the output port of a comparator stage 112, supplied with a reference signal Vref representing the nominal power level Pref for the power Pout of the RF output signal x(t), whose actual output power level is supplied by a directional coupler 106′ and fed back to the APC loop 101 by a wideband detector diode 108, is connected with the gain control input port of a variable-gain power amplifier 105, which controls the output power level Pout of the QAM transmitter 100b. 
The complex-valued analog baseband signal xLP(t) (the complex envelope or equivalent low-pass signal of the real-valued RF band-pass signal x(t) to be transmitted) can thereby be written as follows:
                                          x            LP                    ⁡                      (            t            )                          =                                            ⅈ              ⁡                              (                t                )                                      +                          j              ·                              q                ⁡                                  (                  t                  )                                                              =                                                    a                ⁡                                  (                  t                  )                                            ·                              ⅇ                                  j                  ·                                      φ                    ⁡                                          (                      t                      )                                                                                            ⁢                                                  ⁢            with                                              (        1        )                                                      ⅈ            ⁡                          (              t              )                                :=                      Re            ⁢                          {                                                x                  LP                                ⁡                                  (                  t                  )                                            }                                      ,                            (                  1          ⁢          a                )                                                      q            ⁡                          (              t              )                                :=                      Im            ⁢                          {                                                x                  LP                                ⁡                                  (                  t                  )                                            }                                      ,                            (                  1          ⁢          b                )                                                      a            ⁡                          (              t              )                                :=                                                                                    x                  LP                                ⁡                                  (                  t                  )                                                                    =                                                                                ⅈ                    2                                    ⁡                                      (                    t                    )                                                  +                                                      q                    2                                    ⁡                                      (                    t                    )                                                                                      ,                            (                  1          ⁢          c                )                                                      φ            ⁡                          (              t              )                                :=                                    ∠              ⁢                                                          ⁢                                                x                  LP                                ⁡                                  (                  t                  )                                                      =                          arc              ⁢                                                          ⁢                              tan                ⁡                                  (                                                            q                      ⁡                                              (                        t                        )                                                                                    ⅈ                      ⁡                                              (                        t                        )                                                                              )                                                                    ,                            (                  1          ⁢          d                )            and j:=√{square root over (−1)} is the imaginary unit. Thereby,                i(t) denotes the in-phase (I) component of xLP(t) in time domain,        q(t) denotes the quadrature (Q) signal of xLP (t) in time domain,        a(t) denotes the magnitude component of xLP(t) which is given by the envelope of x(t), and        φ(t) denotes the phase component of xLP(t), which is also the phase component of x(t).        
i(t) and q(t) are directly up-converted from the baseband to an RF band by means of two modulator stages 104a and 104b, respectively, which are driven by a local oscillator 102 providing a high-frequent carrier signal with a sinusoidal waveformci(t)≡c(t):=Ac·cos(2π·fLO·t),  (2a)wherein Ac (in √{square root over (W)}) is the amplitude factor of the carrier signal ci(t) and fLO (in GHz) is the carrier frequency supplied by the local oscillator 102. A Hilbert transformer 104c, connected to one input port of the up-conversion mixer 104a, provides a 90-degree phase shift of the carrier signal ci(t) such that the carrier signal used for a direct up-conversion of the quadrature signal q(t) from the baseband to the RF band is given by
                                          c            q                    ⁡                      (            t            )                          :=                                            A              c                        ·                          cos              ⁡                              (                                                      2                    ⁢                                          π                      ·                                              f                        LO                                            ·                      t                                                        +                                      π                    2                                                  )                                              =                                    -                              A                c                                      ·                                          sin                ⁡                                  (                                      2                    ⁢                                          π                      ·                                              f                        LO                                            ·                      t                                                        )                                            .                                                          (                  2          ⁢          b                )            
Using xLP(t) (or i(t) and q(t), respectively), the modulated RF signal x(t) to be transmitted can thus be written as follows:
                                                                        x                ⁡                                  (                  t                  )                                            =                            ⁢                              Re                ⁢                                  {                                                                                    x                        LP                                            ⁡                                              (                        t                        )                                                              ·                                          ⅇ                                                                                                    +                            j                                                    ·                          2                                                ⁢                                                  π                          ·                                                      f                            LO                                                    ·                          t                                                                                                      }                                                                                                        =                            ⁢                                                                    ⅈ                    ⁡                                          (                      t                      )                                                        ·                                                            c                      l                                        ⁡                                          (                      t                      )                                                                      +                                                      q                    ⁡                                          (                      t                      )                                                        ·                                                            c                      q                                        ⁡                                          (                      t                      )                                                                                                                                              =                            ⁢                                                                    ⅈ                    ⁡                                          (                      t                      )                                                        ·                                      cos                    ⁡                                          (                                              2                        ⁢                                                  π                          ·                                                      f                            LO                                                    ·                          t                                                                    )                                                                      -                                                      q                    ⁡                                          (                      t                      )                                                        ·                  sin                                                                                                                      ⁢                              (                                  2                  ⁢                                      π                    ·                                          f                      LO                                        ·                    t                                                  )                                                                        (        3        )            
Before being transmitted, the obtained RF signal x(t) has to be amplified since a certain output power level Pout is needed to reach a receiver at a certain distance. For this reason, a gain-controlled power amplifier 105 is needed.
Due to the bursted nature of the RF power of a transmitted signal stream in the uplink of a TDMA-based communication system, the output power of a wireless RF transmitter has to be ramped up to an appropriate level or ramped down to zero between different time slots, respectively, such that the RF output power Pout is constant during transmission in order to facilitate time-division multiplexing of different TDMA channels. A certain time before the transmission of the data starts, the mobile terminal increases the transmission power from zero to the desired output power level Pout. This part of the respective time slot TSi is called “ramp up”. After the desired output power level Pout is reached, the transmission of the data starts. This part of the respective time slot TSi is normally referred to as “useful part”. The last part of TSi is then called “ramp down”.
Today, this ramp-up and ramp-down procedure for stabilizing the output power level Pout of a QAM transmitter 100b is realized by means of an APC circuitry 101 according to the state of the art as depicted in FIG. 1b. Thereby, the output port of a comparator stage 112 supplied with a reference signal Vref representing the nominal power level Pref for the power Pout of the RF output signal and the actual output power level Pout is connected to the gain control input port of a variable-gain power amplifier 105 controlling said output power level Pout. The actual output power level Pout is supplied either by direct measurement (as shown in FIG. 1b, where a part of Pout is coupled out by means of a directional coupler 106′ and fed back to the APC loop 101 by means of a wideband detector diode 108) or by indirect measurement (e.g. by measuring the supply current of the power amplifier 105, which is direct proportional to the output power Pout). The measured voltage level VPD proportional to Pout is then compared with the nominal voltage level Vref proportional to the power level Pref. If the actual power level Pout is higher than the power level Pref of the reference signal, the gain GPA of the variable-gain power amplifier 105 is decreased in order to adjust Pout. Vice versa, GPA is increased if Pout is lower than Pref in order to adjust Pout. During the “ramp-up” part the nominal power level Pref is increased, during the “ramp-down” part it is decreased, and during the “useful” part it remains stable. Since the APC loop 101 adjusts the output power level Pout according to the power level Pref of the reference signal, the output power Pout is ramped up or down, respectively, and remains at a predefined level during the “useful” part.
FIG. 2a shows a block diagram 200a illustrating the principle of a conventional closed-loop power control circuitry for stabilizing the radiated power level Pout of a modulated RF signal x(t) to be transmitted at the output port of an integrated variable-gain power amplifier 105, wherein said conventional closed-loop power control circuitry is realized as a current sense loop 101M according to the state of the art (here for the sake of simplicity called current sense APC loop). This current sense APC loop 100M can advantageously be applied to mobile RF transmitters which are equipped with a patch antenna. Said current sense loop 101M is equipped with a microcontroller 202 (μC) having an input port (2) supplied with a reference signal Vref representing the nominal power level Pref for the output power Pout of the RF signal x(t) to be transmitted and a further input port (1) supplied with a signal which is derived from a voltage drop URM at a low-ohm resistor RM serving as a current sensor 204 in the power supply line of the variable-gain power amplifier 105, wherein said voltage drop URM is proportional to the DC supply current IPA of the variable-gain power amplifier 105. The power control signal at the output port of said microcontroller 202 is low-pass-filtered and fed to a first input port (a power control input port) of the power amplifier 105. Moreover, the current sense APC loop 101M comprises a digital signal processing means 201C which is used for providing a reference ramp signal Vramp which serves as said reference signal Vref.
As depicted in FIG. 2b, which shows a technical realization 200b of the above-mentioned current sense APC loop 200a, said microcontroller 202 comprises an operational amplifier 113 which is used for amplifying a signal derived from said voltage drop URM and a comparator stage 112″ having a first input port supplied with a reference signal Vref representing the nominal power level Pref for the output power Pout of the RF signal x(t) to be transmitted as well as a second input port supplied with an amplified version of the signal which is derived from said voltage drop URM.